This invention relates in general to an aqueous inorganic solution for sealing the porosity of metal components prior to the application of functional surface treatments or performance coatings or machining, and more particularly to an aqueous sealing solution of metallic silicates for sealing the porosity of powdered metal and liquid metal cast components, as well as a process for applying the solution to the substrates of the components so as to mitigate the negative efforts of porosity on functional surface treatments, corrosion resistance, and machineability.
The process of manufacturing components of sintered compacted powdered metal traces its roots to a similar process used to fabricate structural carbon components. The process consists of molding metal powder in a die with movable top and bottom punches under tons of mechanically or hydraulically applied pressure to form a part which after being removed from the die cavity is then sintered in an oven at a temperature just below that of the melting point of the metal or alloy powder used in the process. The sintered component thus formed has the shape of the cavity of the die. Such sintered components have appreciable strength dependent upon many factors including the metal powder or powdered alloy used in the process, the density of compaction during the application of pressure to the die, and the sintering temperature. Powdered metal components with increasingly intricate designs are utilized in a broad spectrum of industries including, but not limited to, automotive, heavy truck, lawn and garden equipment, household appliances, power tools, and mechanical power transmission equipment. An advantage of sintered powdered metal parts is that they yield a part with intricate detail requiring little or, at the most, no machining. The cost of such parts is extremely competitive as compared to the same part that may be machined from billet metal.
The most significant disadvantages of powdered metal parts over a machined wrought part include having a porosity that reduces density and results in slightly lower strength and negatively affects secondary machining. Moreover, the porosity negatively affects the application of functional surface treatments including painting, electroplating, electrocoating, performance coating, and the like. Liquid plating solutions and coating materials applied to the substrates of powdered metal parts can form bubbles, blisters, flakes, pinholes, and the like, during thermal curing. Because of these results, the aesthetic appearance of the parts is mostly unacceptable. Further, powdered metal parts have a tendency to develop red oxide or rust in a short period of time unless protected by oil impregnation, phosphate conversion, electroplating, electrocoating, painting, performance coating, or the formation of blue oxide by the steam-treating process.
The intrinsic porosity of a powdered metal part particularly at the surface will cause absorption of phosphate conversion solutions, electroplating solutions, electrocoating baths, and dip or spray applied paints and performance coatings. This absorption makes it extremely difficult to obtain the desired film thickness of later applied functional surface treatments and particularly those thermosetting finishes that require heat for curing as the absorbed wet functional surface treatments will be driven out forming bubbles, blisters, flaking, and other undesirable discontinuities in the surface film.
Heretofore, there have been porosity reducing processes for powdered metal parts manufacturers as well as job shop platers and coaters. One such process includes the vacuum impregnation of the porous surface with a polymer resin either of a heat-curing or anaerobic-curing type. Another porosity reducing process heretofore known is to peen the surface of the part with grit or shot-blasting that results in some degree of porosity reduction. It has also been known to steam treat the surface of powdered metal parts wherein the parts are first heated in a furnace to reach a specific temperature range, and then subjected to super-heated steam which permeates the surface of the parts and forms xe2x80x9cblue iron oxidexe2x80x9d that fills the interstitial voids between the agglomerated metal powdered particles. Each of these processes has its own drawbacks, chief among them being cost.
Resin impregnation is typically the most costly of the above processes and can leave a resin residue on the surfaces that can interfere with subsequent plating and coating adhesion and appearance. Moreover, the polymer resin is temperature limited to approximately 450 degrees F. or less, which is below the cure temperature of a number of performance coatings, thereby preventing the process from being used where such performance coatings are required.
While grit or shot-blasting is effective in some cases at reducing porosity sufficiently to allow application of performance surface treatments, the design features and shapes of more intricate powdered metal components may be so modified that the surface characteristics or part dimensions become altered and unacceptable to design requirements. In this regard the blast media (grit or shot) may become lodged in threads or blind holes on the surfaces of the parts.
While the steam treating process may be effective in sealing porosity of powdered metal parts, the temperature range used in the process can xe2x80x9cdrawxe2x80x9d hardness and impart a degree of brittleness to the surface of the part. Because of this result, steam treating for reducing porosity is avoided where secondary machining of the parts is required. Moreover, the formation of oxide from steam treating can affect the parts dimensionally by making outside diameters larger and inside diameters smaller which can also result in non-conforming parts and resultant scrap cost.
Many of the above porosity problems similarly exist with parts made from casting of liquid iron, steel, aluminum, titanium, magnesium, copper, brass, bronze, zinc, and their alloys, as well as other castable metals and their alloys.
Heretofore, it has been known to treat the surface of porous metallic parts by the process of using aqueous metallic salt solutions applied electrolytically as carriers for the metallic sulfides, as disclosed in U.S. Pat. No. 4,368,107. Such an electrolysis process can lead to hydrogen embrittlement in high-carbon alloy substrates and is therefore undesirable.
U.S. Pat. No. 4,508,681 teaches the application of an alkali metal silicate to a sintered metal substrate in conjunction with conventional heat induced hardening for the advantage of promoting surface sealing and limiting hardening by conventional heat treating operations by blocking the entry into the surface of the part of oxygen, nitrogen or carbon. It is not the objective of teaching in this patent to provide for significant reduction of porosity because it is stated that while only the surface is sealed and hardened internal porosity of the substrate is desired for subsequent intake of lubricants for using the part as a bearing in machinery.
U.S. Pat. No. 4,698,269 teaches porosity sealing of a sintered iron component when treating the component with a solvent carried phosphorizing compound and thereafter applying a chromium containing coating composition. While this process does reduce porosity, it is not as cost-effective as resin impregnation, steam treating and grit or shot blasting. Moreover, the phosphorizing process may not lend itself to subsequent plating and coating operations where acids or electrolysis is employed and the solvent used poses environmental concerns.
Applications of Group 1A metallic silicates are taught in U.S. Pat. Nos. 5,205,874 and 5,672,390 as being useful in protecting wood or metal surfaces from abrasion, corrosion, heat and fire by the formation on the surface of an inorganic insoluble silicon dioxide film. This patent is not concerned with the surface treatment of porous substrates for the purpose of porosity reduction.
It has long been well known to use Group 1A alkali silicates (lithium, sodium, potassium) as binders in coatings containing a variety of fillers. U.S. Pat. Nos. 5,888,280 and 6,287,372 relate to coatings that include a variety of metallic and non-metallic functional pigments.
The aqueous inorganic solution of the present invention and the process for applying the solution to sintered powdered metal parts or castings formed from liquid metal reduces and seals the porosities of these parts, while negligibly affecting the dimensions of the parts and not altering the metallurgical or strength properties of the parts. Accordingly, the presents invention solves the problems heretofore known in the technology of preparing powdered metal and liquid cast parts for efficiently receiving subsequent coatings that become more effective.
Moreover, the porosity reducing solution and method of application according to the present invention essentially eliminates the xe2x80x9cbleedxe2x80x9d or xe2x80x9coutgassingxe2x80x9d of absorbed materials during subsequent plating, coating and machining operations. Further, the solution of the present invention and the process of applying the solution according to the invention permits the application of functional surface treatments in the required thickness without significantly modifying the surface where applied, while improving corrosion protection and machineability and preparing the surface for application of functional plating or coating. It will be further appreciated that the aqueous inorganic solution and method of application according to the present invention is cost-advantageous in that the porosity can be reduced in a timely and economical manner, while being environmentally compliant. The aqueous inorganic solution of the present invention utilizes low cost raw materials that may be combined by simple blending and applied and cured through unique combinations of existing application technologies.
It is therefore an object of the present invention to provide a new and improved aqueous inorganic solution for reducing the porosity of sintered powdered metal parts and cast liquid metal parts so that subsequent coatings can be more effective, and wherein the solution is formulated through the use of low cost raw materials and may be applied by existing application technologies.
Another object of the present invention is to provide an aqueous inorganic solution made from a blend of metallic salt silicates with metal ions taken from Group 1A elements, and a process for applying the solution to sintered powdered metal or liquid cast metal parts that is environmentally compliant and poses no health, safety or environmental hazards to users.
Another object of the present invention is in the provision of an aqueous inorganic solution for application to porous metal parts to reduce porosity that will improve corrosion protection and machineability and prepare the surface for efficient subsequent application of functional plating and coatings.
A still further object of the present invention is in the provision of an aqueous inorganic solution and a process for application of the solution to sintered powdered metal or liquid cast parts for reducing porosity in a cost-advantageous manner, while negligibly changing the dimensions of the parts and not changing the metallurgical or strength properties.
Other objects, features and advantages of the invention will be apparent from the following detailed disclosure, taken in conjunction with the accompanying sheet of drawings, wherein like reference numerals refer to like parts.